This invention relates to a bicarbonate ion-sensitive membrane which is useful in the construction of an ion-selective electrode for measuring the activity of bicarbonate ion in a solution. More particularly, this invention relates to an anion-sensitive membrane which contains an aromatic boric diester structural unit and an onium salt structural unit and which, when it is used as the boundary membrane of an ion-selective electrode, can detect bicarbonate ions (i.e., hydrogencarbonate ions) selectively.
In recent years, attempts are being extensively made to apply ion-selective electrodes to medical fields and thereby determine various ions contained in biological fluids such as blood and urine. Its purpose is to measure the concentrations of specific ions in biological fluids and thereby diagnose various diseases, on the basis of the fact that such ion concentrations are closely related with metabolic reactions in the living body. At present, ion-selective electrodes are being used to measure the concentrations of sodium ion, potassium ion and chloride ion in biological fluids, so that these ion concentrations can be measured conveniently and rapidly.
Generally, as illustrated in FIG. 1, an ion-selective electrode is basically constructed by providing a cylindrical vessel 11 with a barrier membrane comprising an anion-sensitive membrane 12 at the part thereof which is to be immersed in sample solutions (generally at the bottom thereof) and placing therein an internal electrolyte 13 and an internal reference electrode 14.
FIG. 2 illustrates a typical construction of an ion measuring apparatus for measuring the activity of an ion in a solution by using such an ion-selective electrode. Specifically, an ion-selective electrode 21, together with a salt bridge 22, is immersed in a sample solution 23. The other end of the salt bridge, together with an external reference 24, is immersed in a saturated potassium chloride solution 26. The potential difference between both electrodes is read with an electrometer 25, and the ionic activity of a specific ionic species in the sample solution can be determined from the potential difference. The performance of the ion-selective electrode used in such an ion measuring apparatus is greatly affected by the performance of the ion-sensitive membrane used therein.
Hydrogencarbonate ion is one of the important ions present in biological fluids, particularly blood. Since hydrogencarbonate ion is an important factor in revealing the state of respiratory and metabolic functions in the living body, information useful for the diagnosis of various diseases such as diabetes mellitus and renal disorders can be obtained by the measurement of Hydrogencarbonate ion. At present, hydrogencarbonate ion concentrations are calculated from the pH of the sample and the measured value of carbon dioxide partial pressure (PCO2) according to the following equation.
pH=6.1+log{[HCO3xe2x88x92]/0.03xc2x7PCO2}
This method requires a measuring time of as long as about 30 to 60 seconds. Moreover, it is necessary to employ a measuring method different from that for the aforesaid sodium ion-, potassium ion- and chloride ion-selective electrodes. Consequently, it is necessary to measure hydrogencarbonate ion concentrations in a system different from that for the measurement of sodium and potassium ion concentrations.
Generally, ion-selective electrodes can reduce the time required for measurement to the order of several seconds. Moreover, the concentrations of various ions can be simultaneously measured by using a combination of ion-selective electrodes corresponding to ionic species to be measured. Owing to these advantages, a variety of anion-sensitive membranes for the selective detection of hydrogen-carbonate ion have conventionally been proposed. They include, for example,
(a) a membrane obtained by mixing a polymer (e.g., polyvinyl chloride) with a fat-soluble cation salt (e.g, a quaternary ammonium salt), a trifluoroacetophenone derivative (e.g., trifluoro-acetyl-p-alkylbenzene) and a plasticizer, and forming this mixture into a membrane; and
(b) a membrane obtained by mixing a polymer (e.g., polyvinyl chloride) with an organotin compound (e.g., trioctyltin chloride) and a plasticizer and optionally with a trifluoroacetophenone derivative (e.g., trifluoroacetyl-p-alkylbenzene), and forming this mixture into a membrane.
Ion-selective electrodes using the anion-sensitive membrane of the aforesaid type (a) include, for example, an electrode disclosed by Wise et al. (U.S. Pat. No. 3,723,281), an electrode reported by Greenberg et al. [J. Greenberg et al., Anal. Chim. Acta, 141: p. 57-64 (1982)], an electrode disclosed by Chapoteau et al. (Japanese Laid-Open Patent No. 10759/""86=EP-A-155162), and an electrode disclosed by Yamaguchi et al. (Japanese Laid-Open Patent No. 265559/""87=EP-A-245168).
Moreover, ion-selective electrodes using the anion-sensitive membrane of the aforesaid type (b) include an electrode reported by Oesch et al. [J. Oesch et al., J. Chem. Soc. Faraday Trans. 1, 82: p. 1179-1186 (1986)], an electrode disclosed by Ushizawa et al. (Japanese Laid-Open Patent No. 204368/""92), and the like.
However, the ion-selective electrodes using the anion-sensitive membrane of the aforesaid type (a) are known to have poor selectivity relative to fat-soluble ions such as nitrate and thiocyanate ions. Moreover, they also have the disadvantage that their electric potential response is relatively slow (about 1 minute) and their lives are short because the ion-sensitive substance present in the membrane gradually dissolves in the solution.
On the other hand, the ion-selective electrodes using the anion-sensitive membrane of the aforesaid type (b) are known to have poor selectivity relative to chloride ion, because an organotin compound is contained in the membrane. Moreover, they also have the disadvantage that their lives are short because the ion-sensitive substance present in the membrane gradually dissolves in the solution.
Accordingly, it is desired to develop an anion-sensitive membrane capable of yielding an ion-selective electrode which permits hydrogencarbonate ions contained in biological fluids to be rapidly determined with high selectivity and which has a long life.
The present inventors have carried out intensive investigations with a view to developing an anion-sensitive membrane which can solve the above-described problems. As a result, it has now been found that, when a polymer membrane formed by using an onium salt compound and an organic boric diester compound is used as an anion-sensitive membrane, hydrogencarbonate ions present in a solution can be rapidly determined with high selectivity and, moreover, the membrane has a long life. The present invention has been completed on the basis of this finding.
According to the present invention, there is provided a membrane sensitive to bicarbonate ion comprising a polymer membrane which contains an onium salt structural unit (A) and an aromatic boric diester structural unit (B) of the formula

 
wherein Ar is an aromatic carbocyclic group which may optionally have one or more substituents, either in the form of low-molecular-weight compounds dispersed in the polymer or in a form introduced into a polymer molecule.